Method for cutting a work piece

ABSTRACT

A method for cuffing or hard turning the surface of a workpiece comprising using a cutting tool in which a cooling fluid is supplied to the cutting area by means of at least one jet nozzle. The cooling fluid is supplied as a jet with high pressure leaving the jet nozzle with a pressure above 60 bar.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for cutting the surfaceof a workpiece in which a cooling fluid is supplied to the cutting area.More particularly, the invention relates to a method for hard turning ofworkpieces with a Rockwell hardness of 50 HRC or above with ceramic orCBN (cubic boron nitride) tools with a cutting edge having a Vickershardness of 1700 HV or above. In this description the expression “hardturning” means turning of carbon steel through or surface hardened to aRockwell hardness of >50 HRC and/or with martensitic or bainiticmicrostructure. As an example the invention may be used for cutting arace surface of a bearing which is in rolling contact with rollingelements.

BACKGROUND OF THE INVENTION

[0002] Today, all hard turning with ceramic or CBN tools is performeddry or with low-pressure flood cooling. All known recommendations saythat it is very important to keep a high temperature in the cutting zonein order to achieve a good tool life and production economy.

[0003] The research work performed according to the invention shows thatthis is not really true. A primary object of the invention has been toprovide a method of hard cutting by which the tool life is increased,higher cutting speed and a better control of the chip shape areobtained.

SUMMARY OF THE INVENTION

[0004] According to the present invention it has surprisingly shown thatwhen applying a high-pressure jet of cooling fluid just into the gapbetween the clearance face of a tool and the workpiece material, anumber of important advantages are obtained as will be described below.

[0005] The method generally entails using high pressures. The coolingfluid, just before leaving the jet nozzle, having a pressure from 60,preferably 120 to about 400 bar.

[0006] According to one aspect, the present invention provides a methodfor cutting or hard turning the surface of a workpiece using a cuttingtool comprising supplying a cooling fluid to a cutting area, wherein inthat the cooling fluid is supplied as a jet with high pressure leavingthe jet nozzle with a pressure above 60 bar.

[0007] The method according to the invention further includes hardturning of workpieces with a Rockwell-hardness of 50 HRC or above usinga cutting tool with a cutting edge comprising any material being harderthan 1700 HV, especially Al₂O₃ or CBN (cubic boron nitride) with aVicker hardness above 1700 HV, preferably above 2800 HV. Examples ofsuitable cooling fluids include water and carbon dioxide.

[0008] According to another aspect of the invention the cooling fluid isapplied simultaneously to the gap between the chip and a rake face ofthe tool and between the clearance face of the tool and the workmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The above and other advantages of this invention will beappreciated from the following description in connection with thedrawings in which:

[0010]FIG. 1 is a schematic sectional view of a part of the workpieceand tool in hard turning according to the method according to theinvention; and

[0011]FIG. 2 is a schematic sectional view similar to FIG. 1 of a secondembodiment of the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0012] A method is provided for hard turning a workpiece which can leadto significant increases in productivity and production economy.

[0013] As stated above hard turning is a machining operation with ageometrically defined cutting edge on materials that have undergone ahardening treatment. All materials (ferrous and non ferrous) whichcontain sufficient alloy elements (carbon, etc.) that can be hardenedvia a heat treatment process, giving a microstructure which consists ofmartensite and/or bainite with a hardness >50 HRC. These heat treatmentsinclude through hardening, air hardening and surface hardening, forexample, by induction and carburizing treatments. Such workpiecematerials can be hard turned. For ferrous materials mostly CBN or PCBNtools are used for turning. For non-ferrous materials, diamond is used.

[0014]FIG. 1 represents the method according to the invention and showsa workpiece 10 such as a bearing race which is hard turned. Theworkpiece 10 is moving with the speed Vo indicated by the arrow inrelation to a tool 12 cutting a piece of the workpiece material such asa chip 14 with the tool tip. According to this invention, ahigh-pressure jet is applied into the gap between the clearance face ofthe tool insert and the workpiece material by means of a nozzle 16. Afluid wedge 18 is formed between the clearance face of the tool and theworkpiece.

[0015] It has shown that when applying a high-pressure jet just into thegap between the clearance face of a CBN cutting tool and the workpiece,the tool life is increased by more than 100%. The jet should be appliedas perpendicularly as possible to the cutting edge. It has also beenobserved that the residual stresses on the workpiece are slightly morecomprehensive that in conventional low-pressure cooling.

[0016] The pressures used shall be in the range between 6 and 40 MPa(that is between 60 and 400 bar). The highest increase of the tool life(about 110%) was observed for the higher pressure. The jet was createdusing a nozzle with a diameter of 0.7 mm. A conventional syntheticcutting fluid was used. The solution was about 7%.

[0017] According to a second embodiment of the method according to theinvention the high-pressure jet is also applied into the gap between thechip and the tool rake surface as shown in FIG. 2, in which parts whichcorrespond to the parts of the first embodiment have the same numerals.Thus, a nozzle 16 applies a high-pressure jet between the chip 14 andthe tool 12 rake surface forming a fluid wedge 18 between the chip andtool.

[0018] It has been observed that when applying a second jet to the toolrake surface, the chip control was improved. Instead of long andcontinuous chips short, “more convenient” chips were produced. It isespecially important in turning of large rings.

[0019] As seen from the above the method according to the inventioncomprising directing a high-pressure jet in hard turning using ceramicor CBN-inserts leads to a number of important advantages such as:

[0020] Decrease of the edge temperature (30-50%)

[0021] Significant decrease of the tool abrasive wear

[0022] Possibility to use higher cutting speed

[0023] Increased chip forming possibility

[0024] Higher quality of product due to lower wear of the tool and lowertemperature

[0025] Possibility to control the residual stresses on and below theworkpiece surface

[0026] While the present invention has been described by reference tothe above-mentioned embodiments, certain modifications and variationswill be evident to those of ordinary skill in the art. Therefore thepresent invention is to be limited only by the scope and spirit of theappended claims.

I claim:
 1. A method for cutting or hard turning the surface of aworkpiece using a cutting tool comprising supplying a cooling fluid to acutting area, wherein in that the cooling fluid is supplied as a jetwith high pressure leaving the jet nozzle with a pressure above 60 bar.2. The method according to claim 1 , wherein the cooling fluid jet has apressure above 120 bar.
 3. The method according to claim 1 , wherein thecutting comprises hard turning of workpieces with a Rockwell hardness of50 HRC or above using a cutting tool with a cutting edge having aVickers-hardness of 1700 HV or above.
 4. The method according to any ofthe claim 1 , wherein the cutting tool comprises a cutting edge formedof cubic boron nitride with a Vickers hardness above 1700 HV.
 5. Themethod according to claim 1 , wherein the cooling fluid comprises water.6. The method according to claim 1 , wherein the cooling fluid comprisescarbon dioxide.
 7. The method according to claim 1 , wherein the coolingfluid is supplied between a clearance face of the tool and theworkpiece.
 8. The method according to claim 1 , wherein the coolingfluid is supplied to a gap between the chip and a rake face of the tool.9. The method according to claim 1 , wherein cooling fluid issimultaneously applied to a gap between the chip and a rake face of thetool and a clearance face of the tool and the workpiece material. 10.The method of claim 4 , wherein the hardness is above 2800 HV.